Global change and invasive jellyfish: Hypoxia, fish predation and ecosystem effects

Coastal marine ecosystems are highly productive and biodiverse, playing key roles in nutrient cycling and supporting fisheries—functions essential for sustainable societies but increasingly impacted by human activities. This thesis explores the effects of anthropogenic influences on coastal areas, emphasizing the alterations caused by an invasive jellyfish and low oxygen levels (hypoxia). Global warming and nutrient discharges have led to increased primary production; when this excess production reaches the bottom, it fuels hypoxia, globally modifying coastal ecosystems and favoring species tolerant to hypoxia. Among these tolerant species is the comb jelly Mnemiopsis leidyi, which has invaded several regions where hypoxia is prevalent and human activity is high. In these environments, M. leidyi competes for food with native jellyfish and fish. Fish are less tolerant of hypoxia than jellyfish and are expected to decline in distribution. Meanwhile, fish might be the only non-gelatinous predators of M. leidyi.

To better understand the dynamics of these altered systems, this thesis investigates how M. leidyi is impacted by anthropogenic stressors and investigates whether M. leidyi has predators among fish species. Initial field observations established the distribution of M. leidyi life stages in the Baltic Sea, where a lack of response to hypoxia was observed. Consequently, experiments were designed to investigate the thresholds and processes underlying this tolerance. The experiments revealed that M. leidyi maintains high feeding and reproduction levels even in severe hypoxia and when simultaneously exposed to crude oil, demonstrating greater robustness than its fish competitors.

To determine whether M. leidyi is preyed upon by fish, various Baltic Sea fish species were exposed to different sizes of M. leidyi. Several tested fish species captured but refused to eat M. leidyi. However, the increasingly abundant three-spined stickleback did ingest M. leidyi. Yet, when exposed to moderate hypoxia (30% O₂ saturation), the predation effect was reduced by 68%. A similar reduction was observed when sticklebacks fed on their shared prey, the copepod Acartia tonsa.

These results imply that between 30% and near-anoxic conditions (4% O₂ saturation), M. leidyi will thrive while experiencing minimal predation and competition from fish. Its predation on copepods drastically reduces prey availability for fish, altering coastal ecosystem functioning with wide and long-lasting consequences. Studies have documented improved conditions, including reductions in M. leidyi populations, where nutrient discharges are reduced. This demonstrates that efforts to combat global warming and reduce nutrient inputs can mitigate the detrimental developments of past decades. To reestablish resilient ecosystems and sustain thriving societies, restoring previously high oxygen levels is essential.